Usnea

Western herbalism uses the whole thallus, treats the species as one remedy, and classes it as an antimicrobial, a vulnerary and a respiratory remedy.

It is given for sore throat, sinus congestion and bronchial infection, for urinary-tract irritation, for fungal overgrowth and for infected wounds. Practitioners match it to infection that sits alongside inflammation, stagnant mucus or a damp tissue state.

Internally it goes into respiratory and urinary formulas, usually as a tincture. Externally it is applied to wounds directly.

Usnea has been used in traditional medicine around the world ^{1Reference 1Sepahvand et al. · 2021ReviewUsnea sp.: antimicrobial potential, bioactive compounds, ethnopharmacological uses and other pharmacological properties — reviewView study →}, and alongside that as a dye, a cosmetic, a preservative and a deodorant ^{35Reference 35Salgado et al. · 2017Secondary metabolite profiling of species of the genus Usnea by UHPLC-ESI-OT-MS-MS — analyticalView study →}. Folk practice uses it two ways: externally on wounds, and internally for infections and lung complaints.

The dry lichen was also packed straight into wounds as an emergency dressing.

Across many independent in vitro studies, usnic acid inhibits Gram-positive bacteria, including Staphylococcus aureus, S. epidermidis, Enterococcus faecalis and E. faecium ^{4,6Reference 4Tozatti et al. · 2016In vitroActivity of the lichen Usnea steineri and its major metabolites against Gram-positive, multidrug-resistant bacteria — in vitroView study →Reference 6Francolini et al. · 2004In vitroUsnic acid, a natural antimicrobial agent able to inhibit bacterial biofilm formation on polymer surfaces — in vitroView study →}. Against multidrug-resistant strains, (+)-usnic acid from Usnea steineri had an MIC of 3.12 µg/mL for S. epidermidis and 12.5 µg/mL for S. aureus; diffractaic acid from the same lichen was inactive ^{4Reference 4Tozatti et al. · 2016In vitroActivity of the lichen Usnea steineri and its major metabolites against Gram-positive, multidrug-resistant bacteria — in vitroView study →}.

Cansaran and colleagues found that the antimicrobial activity of Usnea acetone extracts rose with their usnic acid content, which ranged from 0.22% to 6.49% of dry weight across six species ^{3Reference 3Cansaran et al. · 2006In vitroIdentification and quantitation of usnic acid from Usnea species of Anatolia and antimicrobial activity — in vitroView study →}.

In methicillin-resistant S. aureus, usnic acid damaged peptidoglycan (cell-wall) synthesis and impaired adhesion and biofilm formation ^{5Reference 5Pompilio et al. · 2016In vitroAntibacterial and antibiofilm mechanisms of usnic acid against methicillin-resistant Staphylococcus aureus — in vitroView study →}. Loaded into polyurethane, it prevented S. aureus biofilm from forming on the surface ^{6Reference 6Francolini et al. · 2004In vitroUsnic acid, a natural antimicrobial agent able to inhibit bacterial biofilm formation on polymer surfaces — in vitroView study →}.

Gram-negative bacteria are more often resistant to Usnea extracts ^{1Reference 1Sepahvand et al. · 2021ReviewUsnea sp.: antimicrobial potential, bioactive compounds, ethnopharmacological uses and other pharmacological properties — reviewView study →}, and Pseudomonas aeruginosa still formed biofilms on usnic acid-loaded polymer ^{6Reference 6Francolini et al. · 2004In vitroUsnic acid, a natural antimicrobial agent able to inhibit bacterial biofilm formation on polymer surfaces — in vitroView study →}. In one disk-diffusion study, usnic acid from U. longissima did inhibit E. coli as well as S. aureus ^{7Reference 7Maulidiyah et al. · 2020In vitroAntibacterial activity of usnic acid from Usnea longissima Ach. — in vitroView study →}.

Gap: Every result is in vitro; there are no animal infection models or human trials, and because usnic acid is poorly water-soluble the relevant activity belongs to alcoholic extracts and isolated compound, not a water infusion.

Usnic acid acts on yeast biofilms and virulence traits in vitro. It inhibited Candida albicans biofilm formation by 65%, blocked the switch from yeast to hyphal form, thinned mature biofilms and reduced the sugars in the biofilm matrix ^{9Reference 9Nithyanand et al. · 2015In vitroUsnic acid inhibits biofilm formation and virulent morphological traits of Candida albicans — in vitroView study →}.

It was also active against C. parapsilosis and C. orthopsilosis, reducing the activity of biofilm cells by 80% ^{8Reference 8Pires et al. · 2012In vitroEffect of usnic acid on Candida orthopsilosis and C. parapsilosis — in vitroView study →}. Against azole-resistant C. albicans biofilms it raised oxidative and nitrosative stress inside the biofilm ^{10Reference 10Peralta et al. · 2017In vitroUsnic acid activity on oxidative and nitrosative stress of azole-resistant Candida albicans biofilm — in vitroView study →}.

Whole Usnea extracts have more often been inactive against Candida ^{1Reference 1Sepahvand et al. · 2021ReviewUsnea sp.: antimicrobial potential, bioactive compounds, ethnopharmacological uses and other pharmacological properties — reviewView study →}. There are no in vivo antifungal studies.

(+)-Usnic acid had dose-dependent anti-inflammatory activity in acute and chronic rat models ^{11Reference 11Vijayakumar et al. · 2000AnimalAnti-inflammatory activity of (+)-usnic acid — rat modelView study →}. In mice with LPS-induced acute lung injury it improved survival and lung oedema, and lowered inflammatory cells, inflammatory cytokines and oxidative-stress markers in the lungs ^{12Reference 12Su et al. · 2014AnimalUsnic acid protects against LPS-induced acute lung injury in mice through attenuating inflammatory responses and oxidative stress — mouse modelView study →}.

In an MPTP mouse model of Parkinson’s disease, usnic acid at 5 or 25 mg/kg reduced motor problems and nerve-cell loss, and in astrocytes it blocked glial activation by inhibiting NF-κB ^{14Reference 14Lee et al. · 2020AnimalAnti-inflammatory effects of usnic acid in an MPTP-induced mouse model of Parkinson’s disease — mouse modelView study →}.

A 2023 review concluded that usnic acid has real anti-inflammatory potential, while noting the limits of the studies and the need to check its safety ^{13Reference 13Paździora et al. · 2023ReviewCritical assessment of the anti-inflammatory potential of usnic acid and its derivatives — reviewView study →}.

These studies used isolated usnic acid, not the whole lichen.

Usnic acid was active against both drug-susceptible and drug-resistant Mycobacterium tuberculosis and against non-tuberculous mycobacteria, and efflux pumps played no part in resistance to it ^{15Reference 15Ramos et al. · 2010In vitroAntimycobacterial activity of usnic acid against resistant and susceptible strains of Mycobacterium tuberculosis and non-tuberculous mycobacteria — in vitroView study →}.

Among lichen compounds tested against M. tuberculosis, the depside diffractaic acid was the most active (MIC 15.6 µg/mL), followed by norstictic acid and usnic acid (both 62.5 µg/mL) ^{16Reference 16Honda et al. · 2010In vitroAntimycobacterial activity of lichen substances — in vitroView study →}.

Usnic acid treatment remodels the mycobacterial cell envelope and disturbs the bacterium’s redox balance ^{17Reference 17Sieniawska et al. · 2021In vitroUsnic acid treatment changes the composition of Mycobacterium tuberculosis cell envelope and alters bacterial redox status — in vitro. mSystems. https://pubmed.ncbi.nlm.nih.gov/33947802/View study →}.

Gap: Activity is entirely in vitro at concentrations near or above those that are hepatotoxic in mammalian cells; no animal efficacy or clinical work exists.

Sodium usnic acid applied to skin wounds in rats sped wound closure and re-epithelialisation, with fewer inflammatory cells and more fibroblasts, new blood vessels and organised collagen than untreated controls ^{18Reference 18Zhang et al. · 2018AnimalThe effects of sodium usnic acid by topical application on skin wound healing in rats — rat modelView study →}. A body of formulation work — films, scaffolds, dressings and nanocarriers — builds on usnic acid’s combined antibacterial and healing effects for infected and burn wounds ^{19Reference 19Francolini et al. · 2019ReviewUsnic acid: potential role in management of wound infections — reviewView study →}.

Topical use keeps systemic exposure low.

The in vivo evidence is a handful of rodent studies, and most recent papers test delivery materials rather than healing. No clinical wound trial exists.

A methanol extract of lab-cultured Usnea ghattensis prevented lipid peroxidation and scavenged superoxide, DPPH, nitric oxide and hydroxyl radicals at levels higher than the synthetic antioxidants BHT and BHA at the same concentration. It also protected mouse liver slices against ethanol damage ^{20Reference 20Verma et al. · 2008In vitroAntioxidant and hepatoprotective activity of a lichen Usnea ghattensis in vitroView study →}. An earlier study of U. ghattensis solvent extracts also found antioxidant activity, and the extracts inhibited Bacillus species and S. aureus (MIC 5–10 µg/mL) ^{21Reference 21Behera et al. · 2005In vitroAntioxidant and antibacterial activities of lichen Usnea ghattensis in vitroView study →}.

A comparison of lichen species found strong antioxidant activity in Usnea longissima methanol extract, though its activity did not track its phenolic content ^{22Reference 22Odabasoglu et al. · 2004In vitroComparison of antioxidant activity and phenolic content of three lichen species — in vitroView study →}.

All of it is in vitro and extract-based, with no in vivo confirmation.

Usnic acid is toxic to many cancer cell lines, but it dissolves poorly and injures the liver. A water-soluble salt, potassium usnate, reached higher levels in tumour, liver and plasma than usnic acid, and inhibited invasion and liver metastasis in an orthotopic mouse colorectal cancer model ^{23Reference 23Yang et al. · 2018AnimalPotassium usnate, a water-soluble usnic acid salt, shows enhanced bioavailability and inhibits invasion and metastasis in colorectal cancer — mouse modelView study →}.

In HeLa cervical cancer cells, usnic acid lowered PD-L1 through the STAT3 and RAS pathways and enhanced T-cell killing ^{24Reference 24Sun et al. · 2021In vitroUsnic acid suppresses cervical cancer cell proliferation by inhibiting PD-L1 expression and enhancing T-lymphocyte tumour-killing activity — in vitroView study →}. (+)-Usnic acid inhibited the motility of non-small-cell lung cancer cells as effectively as the crude lichen extract ^{25Reference 25Yang et al. · 2016In vitroInhibitory activity of (+)-usnic acid against non-small-cell lung cancer cell motility — in vitroView study →}.

Reviews of lichen metabolites name usnic acid as the lead anticancer candidate ^{26Reference 26Tripathi et al. · 2022ReviewA review of anti-cancer and related properties of lichen extracts and metabolites — reviewView study →}.

Gap: Almost all evidence is cell-line based; the one in vivo study used a chemically modified salt, not the native lichen, and the dose window is constrained by hepatotoxicity.

Usnic acid inhibited three strains of SARS-CoV-2 (Wuhan, Delta and Omicron) in vitro, and two of a series of synthetic derivatives were also highly active ^{27Reference 27Filimonov et al. · 2022In vitroFilimonov, A. S., et al. (2022). (+)-Usnic acid and its derivatives as inhibitors of a wide spectrum of SARS-CoV-2 viruses — in vitro. Viruses. https://pubmed.ncbi.nlm.nih.gov/36298709/View study →}.

In a separate study, (+)-usnic acid was picked out by computer screening and then inhibited SARS-CoV-2 in cell culture with an IC50 of 7.99 µM, similar to remdesivir; its sodium salt did slightly better (5.33 µM) ^{28Reference 28Oh et al. · 2022In vitroOh, E., et al. (2022). (+)-Usnic acid and its salts, inhibitors of SARS-CoV-2, identified by using in silico methods and in vitro assay. Scientific Reports. https://pubmed.ncbi.nlm.nih.gov/35908082/View study →}.

Gap: All of this is in cell culture or on computer models; there is no animal or clinical evidence, and the concentrations have not been compared with what a usnea preparation delivers.

Liver. Usnic acid uncouples mitochondrial energy production, depletes ATP and glutathione, and causes oxidative injury to liver cells ^{29,30Reference 29Kwong et al. · 2020ReviewReview: usnic acid-induced hepatotoxicity and cell death — reviewView study →Reference 30Han et al. · 2004In vitroUsnic acid-induced necrosis of cultured mouse hepatocytes: inhibition of mitochondrial function and oxidative stress — in vitroView study →}. Since 2000 the US FDA has received more than 20 reports of severe illness after usnic acid-containing supplements ^{29Reference 29Kwong et al. · 2020ReviewReview: usnic acid-induced hepatotoxicity and cell death — reviewView study →}. In 2001 it issued a warning about one such product, LipoKinetix, which was then withdrawn from the market ^{32Reference 32Chen et al. · 2025ReviewHepatotoxicity of usnic acid and underlying mechanisms — reviewView study →}. At least two people needed a liver transplant after taking usnic acid for weight loss, one of them on pure usnic acid ^{36,37Reference 36Durazo FA et al. · 2004Case reportFulminant liver failure due to usnic acid for weight loss — case reportView study →Reference 37Sanchez W et al. · 2006Case reportSevere hepatotoxicity associated with use of a dietary supplement containing usnic acid — case reportView study →}.

Whole lichen. In three-month feeding studies, ground Usnea lichen damaged the liver in male rats from 120 ppm usnic acid, in female rats at 720 ppm, and in mice at 360 ppm ^{31Reference 31National Toxicology Program · 2022AnimalToxicity studies of Usnea lichens containing (±)-usnic acid administered in feed to F344/N rats and B6C3F1 mice — animal studyView study →}. Purified (+)-usnic acid was tested the same way ^{34Reference 34National Toxicology Program · 2022AnimalToxicity studies of (+)-usnic acid administered in feed to F344/N rats and B6C3F1 mice — animal studyView study →}. Traditional use is short-term and at far smaller amounts than concentrated supplements.

Skin. Patients using a lichen-based deodorant developed allergic contact dermatitis and reacted to usnic acid on patch testing ^{41Reference 41Sheu M et al. · 2006Case reportAllergic contact dermatitis from a natural deodorant: a report of 4 cases associated with lichen acid mix allergy — case seriesView study →}, and usnic acid in vaginal pessaries has also caused contact dermatitis ^{42Reference 42Rafanelli S et al. · 1995Case reportContact dermatitis from usnic acid in vaginal ovules — case reportView study →}.

Other medicines. No interaction study exists. Take care alongside alcohol, paracetamol (acetaminophen) or other substances that stress the liver.